In typical fuel cells that are in use at the moment, a cathode area is separated by an electrolyte from an anode area, with the electrolyte that is used varying depending on the fuel cell type. For example, a proton-conductive membrane is used as the electrolyte in polymer electrolyte fuel cells. During operation of the fuel cell, hydrogen is supplied to the anode side and an oxidant containing oxygen, for example air, is supplied to the cathode side. The hydrogen molecules react at an anode catalytic converter, which is provided in the anode area, in accordance with the equationH2→2·H++2·e−
and in the process emit electrons to the electrode, forming positively charged hydrogen ions.
The H+-ions which are formed in the anode area then diffuse through the electrolyte to the cathode where they react on a cathode catalytic converter, which is provided in the cathode area, with the oxygen that is supplied to the cathode and with the electrons which have been passed to the cathode via an external circuit, in accordance with the equation0.5·O2+2·H++2·e−→H2O
to form water.
In all known fuel cell systems, water is thus formed as the reaction product of the electrochemical processes in the fuel cell. Furthermore, in some circumstances, additional water is introduced into the fuel cell system together with the reactants to be supplied to the anode area and/or to the cathode area. For example, moisture may be contained in the anode gases to be supplied to the anode area or else in the oxidant, for example air, to be supplied to the cathode area.
However, liquid water can block flow channels that are provided in the fuel cell system, in particular in the area of the fuel cell, and thus adversely affect the uniform distribution of the gaseous reactants in the system and in the fuel cell. This can have a negative effect on the extent of the reaction of the reactants in the fuel cell, thus having a negative effect on the efficiency of the overall system. Furthermore, liquid water in the fuel cell system can freeze at low temperatures, and this can lead to damage to the system components. Because of this, known fuel cell systems normally have liquid separators arranged at critical points in the system, in order to remove liquid water from the system.
By way of example, U.S. Pat. No. 5,200,278 describes a fuel cell system having a fuel cell, an anode gas supply line, an anode gas recirculation line, a cathode gas supply line and a cathode gas output line. A liquid separator with a reservoir for holding liquid water is provided in each case in the cathode gas output line and in the anode gas recirculation line. The water that is collected in the reservoir is periodically carried away from the reservoir. For this purpose, each water separator has a water output line, which is connected to the reservoir of the water separator, and a valve arranged in this water output line. When the water level in the reservoir of the water separator exceeds a predetermined height, the valve is opened, so that the water that has been collected in the reservoir can be carried away from the fuel cell system via the water output line.
A further fuel cell system 10, which is known from the prior art, is illustrated in FIG. 1. The fuel cell system 10 has a fuel cell 12 with an anode area 14, which is separated from a cathode area 18 by an electrolyte 16. Although only a single fuel cell 12 is shown in FIG. 1, the fuel cell system 10 has a plurality of fuel cells 12, which are stacked one above the other to form a so-called fuel cell stack.
The cathode area 18 of the fuel cell 12 is supplied with air via a cathode gas supply line 22, with the aid of a compressor 20. The cathode area 18 of the fuel cell 12 is also connected to a cathode gas output line 24 in order to emit cathode exhaust gases. A moisturizing system 25 is connected to the cathode gas supply line 22 and to the cathode gas output line 24 and, for example, is in the form of a gas-to-gas membrane moisturizer.
In contrast, the anode area 14 of the fuel cell 12 is connected to an anode gas supply line 26, through which hydrogen is supplied to the anode area 14. A first valve 28 is arranged in the anode gas supply line 26 in order to control the hydrogen supply to the anode area 14. An anode gas output line 30 is in the form of a recirculation line, in which a recirculation fan 32 is arranged, in order to convey anode exhaust gases, which emerge from the anode area 14 of the fuel cell 12, in the circuit. Sensors 34, 36 are provided in the anode gas output line 30, which is in the form of a recirculation line, in order to measure the pressure, the temperature and the relative humidity of the anode exhaust gases, as well as the hydrogen concentration in the anode exhaust gases. Furthermore, the anode gas output line 30 is connected via a second valve 38 to a purging line 40. Once the fuel cell system has been switched off, the second valve 38 can be opened, and the anode area 14 of the fuel cell 12 can be purged with a purging gas, for example air, which is supplied through the purging line 40.
In order to prevent liquid water from entering the anode area 14 or the cathode area 18 of the fuel cell 12, and blocking reactor flow channels in this area, liquid separators 42, 44 are in each case arranged in the anode gas supply line 26 and in the cathode gas supply line 22. A further liquid separator 46 is provided in the anode gas output line, in order to protect downstream components, for example sensors 34, 36 and the recirculation fan 32, against damage caused by freezing water at low temperatures.
Each liquid separator 42, 44, 46 has a reservoir 48, 50, 52 and is equipped with a level sensor 54, 56, 58 for measurement of the liquid level in the reservoir 48, 50, 52. A liquid outlet 60, 62, 64 of each liquid separator 42, 44, 46 is connected to a liquid output line 66, 68, 70, in each of which a valve 72, 74, 76 is provided. When a level sensor 54, 56, 58 indicates that the water level in the reservoir 48, 50, 52 of the associated liquid separator 42, 44, 46 has reached a predetermined height, the valve 72, 74, 76 associated with this liquid separator 42, 44, 46 is opened, so that the water which has been collected in the reservoir 48, 50, 52 can be carried away from the fuel cell system 10 through the liquid outlet 60, 62, 64 and the liquid output line 66, 68, 70.
In the known fuel cell system 10 that is illustrated in FIG. 1, each liquid separator 42, 44, 46 is equipped with a separate arrangement, which comprises the level sensor 54, 56, 58, the valves 72, 74, 76 as well as appropriate control components for driving the valves 72, 74, 76, for periodically emptying the water out of the reservoir 48, 50, 52 of the liquid separator 42, 44, 46 in a controlled manner. The system therefore has a multiplicity of individual components, which may be susceptible to defects, and in consequence has a relatively complex overall structure.